Power supply systems are required to operate reliably over a broad range of conditions. These conditions include variations in both input voltages and output load demands. When several power supplies are operated in parallel and derive their power from a single input voltage source, the addition or deletion of one or more of the power supplies may induce a temporary excursion in the input voltage. Additionally, as the output load demands change abruptly on one or more of the paralleled power supplies, the new condition may cause the corresponding voltage operating conditions associated with that power supply to also change temporarily. Also, a fault condition may develop in one or more of the power supplies that requires permanent isolation of those power supplies to protect the others, until the fault is corrected. Each of these situations has the potential to create a transient condition.
Another transient condition occurs when a power supply is first turned on. When the power supply is connected to an input voltage source, the filter capacitors in the power supply are charged. The initial current that flows into the power supply may be quite large compared to the current that the power supply normally accommodates. This temporary "inrush" current must be handled in a manner that prevents damage from occurring to the components of the power supply. Additionally, it is desirable that the inrush current be accommodated in a way that does not affect the overall operating efficiency of the power supply when operating in a steady-state or non-transient mode.
Circuits that limit the magnitude of an inrush current often comprise both passive and active components. Each of these components has a nominal value or set of characteristics that describe the component. Additionally, each component typically has a tolerance stated for its value or characteristic, normally given as a percentage range. The component tolerances are normally selected to assure that the power supply performs correctly during steady-state operation. However, since the inrush current may be large compared to normal operating currents, the component tolerances may allow too large a magnitude range for the inrush current. This causes component damage, unless the critical components are either over-designed or hand-selected. This tends to raise the cost associated with protecting the power supply.
Accordingly, what is needed in the art is a way to minimize the effect of component tolerances on the magnitude of inrush current without affecting overall power supply efficiency.